1. Field of the Invention
The present invention relates to the field of high-performance printed circuit board (PCB) design and more particularly to via stub and connector performance management in PCB design.
2. Description of the Related Art
Interconnects are PCB design structures intended to reliably transmit high-speed serial signals from component to component in a PCB. Transmission lines, vias, and connectors form the basic building blocks of the interconnect design. Each of the transmission lines, vias and connectors can be configured at design time to satisfy desired metrics. Subsequently, the combination can be physically assembled into a PCB system-level interconnect. The most common PCB transmission structures include microstrip and stripline transmission lines. Single-ended lines also have been successfully utilized for lower speed PCB designs, whereas modern gigabit designs utilize differential signaling due to associated substantial noise immunity and reliable current return path characteristics.
Vias are integral to the PCB design in that vias facilitate the routing of circuit traces between layers of a multilayer board. Vias have proven particularly useful for transitioning from the pins of a ball grid array or connector down to stripline traces within the PCB. The most common and inexpensive via structure is the “through-hole” via, while alternatives to the through-hole via include the blind via and the back-drilled via. Although the blind via and the back-drilled via generally provide higher performance, most high-volume designs use the lower cost through-hole via. Importantly, central to the utilization of the PCB via structure is the unterminated via stub.
In this regard, PCB designs can succeed or fail depending on the choice of route layer and resultant via stub length. Specifically, when supporting a branched point-to-point signal path, a via stub can form a quarter wave resonant circuit where one end of the stub contains an open high impedance discontinuity while the other end of the stub contains a low impedance discontinuity where the trace and the via structure meet. The impedance mismatch can form a resonant structure in which a signal reflects back and forth between the boundaries of the structure. As a result, the reflection can remove energy from the signaling path at high frequencies causing a resonant trough at a specific frequency. The resonance caused by the stub can be detrimental to signal quality and whole PCB designs have been ruined by improper via design and routing.